The document summarizes two recent studies on photoluminescence (PL) spectroscopy of copper zinc tin sulfide (CZTS) materials for solar cell applications. The first study examines PL and internal quantum efficiency measurements of CZTSSe and CIGSSe solar cell devices, finding evidence that band tailing from electrostatic potential fluctuations is more severe in CZTSSe. The second study analyzes temperature- and intensity-dependent PL of CZTS polycrystals with varying compositions, observing different emission behaviors correlated with composition and attributing results to a model of fluctuating electrostatic potentials. Both papers provide support for CZTS being a highly disordered system affected by charged defects and potential fluctuations.

1. WMD JOURNAL CLUB
Overview of Recent CZTS Photoluminescence Studies
13th October 2015
Suzanne Wallace

2. Why Would We Care About PL Spectroscopy?
• Easy measurement
Contactless method  does not require a full PV device to be
constructed (…can be time consuming)
• Non-destructive optical technique
• Particularly relevant to PV sub-group (or anyone who cares
about defects in materials!)
• Detecting point defects and bulk impurities in
semiconductors

3. Why Would We Care About PL Spectroscopy?
• Easy measurement
Contactless method  does not require a full PV device to be
constructed (…can be time consuming)
• Non-destructive optical technique
• Particularly relevant to PV sub-group (or anyone who cares
about defects in materials!)
• Detecting point defects and bulk impurities in
semiconductors

4. useful!
+

6. What Is Luminescence?
• ‘Cold light’ (as opposed to incandescence from e.g. a bulb
or flame)
• Light-emission from an excited semiconductor
• Excited by…
• Electric current  electroluminescence (LEDs)
• Electron beam  cathodoluminescence
• Mechanical action  triboluminescence
• Nuclear radiation  radioluminescence
• Optical absorption  photoluminescence
• (and even more types of luminescence!)

7. 2 Types of Photoluminescence
• Fast fluorescence (~ 10-8 s)
• Small dye molecules and nano-inorganic
materials
• Slower phosphorescence (s, mins,
…hours)
• Relatively large-scale inorganic materials
Main difference between
measuring fluorescence and
phosphorescence  delay
between excitation and
measurement

8. What is PL Spectroscopy?
• Optical excitation of a sample
to create e--h+ pairs (excitons)
• Excitons eventually deactivate
either radiatively (give off a
phonon) or non-radiatively
• Radiative recombinations are
observable with high sensitivity
• Lifetime of exciton depends on
crystal lattice, type and
concentration of impurities

9. Common PL Transitions

10. Common PL Transitions
Rarely seen in materials
with small effective masses

11. Common PL Transitions
Free exciton recombination
(materials with strong excitonic
binding show decrease in gap)

12. Common PL Transitions Excitons bound to shallow
donors or free electron to
shallow acceptors

13. Band Tails In Heavily-Doped Semiconductors
• Band structure characterizes
material
• Doping deforms the band
structure
• Each impurity ion (or intrinsic
defect) locally introduces
distinct level in band gap
At high densities local levels
interact to form a band
• For high n-type doping,
impurity band merges with CB
 ‘rigid shift’ of the CB towards VB

14. 1) IQE & PL on CZTSSe Devices: Gokmen,
Mitzi, et al
• IQE & PL on hydrazine process CZTSSe and CIGSSe solar
cells (finished cells and bare films)
• Same behaviours observed for each
• IQE  estimate of band gap
• IQE & PL  band-edge tail states twice as severe in CZTSSe
• Low temp time-resolved PL  tailing primarily from
fluctuations in electrostatic potential

15. IQE & PL On CZTSSe & CIGSSe Devices
• Estimate band gap from inflection of IQE curve
• PL peaks shifts to lower E than Eg (red shift) due to band
tailing from
• Band gap fluctuations (different phases)
• And/ or electrostatic potential fluctuations (charged defects, e.g.
[CuZn- + ZnCu+] complex with lowest formation E)

16. Low Temperature Time-Resolved PL (TRPL)
• Larger difference at low T for CZTSSe than CIGSSe
• Conclude more severe tailing in CZTSSe
• Larger amplitude in electrostatic potential fluctuations due to lower
dielectric constant

17. Band Gap & Electrostatic Potential
Fluctuations
• Don’t rule out band
gap fluctuations
• Link TRPL data to
electrostatic potential
fluctuations
• Diagram a bit
misleading?
Appearance of photon-
assisted transition?

18. 2) PL on CZTS Polycrystals of Differing
Compositions: Halliday, Durose et al
• Temperature- and intensity-dependent PL
• CZTS polycrystals with a range of different compositions
• Reports of Cu-poor compositions giving better PV performance
• Confirm crystalline CZTS using XRD
• Determine compositions from initial weight of elements
• PL peak positions and intensity consistent with model of
fluctuating potential for a heavily doped semiconductor

19. General Overview
• Very good introduction covering key issues for CZTS
• TW scale energy production
• Record efficiencies for devices synthesized by different methods
• Secondary phases
• Ambiguity in interpretation of PL data (DAP emission vs. model of
fluctuating potentials)
• Band tailing introduced nicely

20. General Overview
• Very good introduction covering key issues for CZTS:
• TW scale energy production
• Record efficiencies for devices synthesized by different methods
• Secondary phases
• Ambiguity in interpretation of PL data (DAP emission vs. model of
fluctuating potentials)
• Band tailing introduced nicely

21. General Overview
• Very good introduction covering key issues for CZTS:
• TW scale energy production
• Record efficiencies for devices synthesized by different methods
• Secondary phases
• Ambiguity in interpretation of PL data (DAP emission vs. model of
fluctuating potentials)
• Band tailing introduced nicely
• Mix the two effects discussed separately by Mitzi &
Gokmen?
• Model of fluctuating potentials related to compositional differences
(phases)
• In Mitzi paper this is related to band gap fluctuations

22. Fluctuating Potential Model

23. XRD On 8 CZTS Polycrystals
• Reflections are those
expected for kesterite
CZTS
• Stannite and kesterite
phases differ in Cu and
Zn ordering
• XRD cannot distinguish
between Cu & Zn
• Also considerable overlap
with other phases (e.g.
ZnS)
• Additional peak for
sample C8 attributed to
unidentified secondary
phase

24. PL Data Categorized Into 3 Groups
• Group a: C4, C5, C6  single dominant CZTS feature at 1.4 eV
• Group b: C1, C2, C7  dominant CZTS feature and series of
less intense features
• Group c: C3, C8  emission peaks across full PL range
Results suggest
PL provides
more sensitive
measure of the
impact of
structural
differences and
presence of
secondary
phases in CZTS
than XRD
N.B. Differences in PL data for C1, C2 with C3 which all had same metal composition
but differing excess of S

25. Compositions of CZTS Polycrystals
• C1, C2 and C3 all considered stoichiometric
• Group a: C4, C5, C6
• Group b: C1, C2, C7
• Group c: C3, C8

26. Temperature-Dependent PL
To excite electron or
hole out of local
fluctuating potential
well

27. Temperature-Dependent PL
To excite electron or
hole out of local
fluctuating potential
well
It’s all a
mystery!

28. Temperature-Dependent PL
To excite electron or
hole out of local
fluctuating potential
well
It’s all a
mystery!
• Discussion about
validity of model
• Changes in capture
cross section of one of
the levels provides
alternative explanation
(see ref 47)

29. Main Conclusions From 2 Studies
1. Gokmen, Mitzi et al
• Existence of tail-states due to electrostatic potential fluctuations
are an important factor for CZTS devices
• Do not discount band gap fluctuations but believe effect of potential
fluctuations is more pronounced in CZTS than CIGS due to formation of
charged [CuZn- + ZnCu+] defects and low dielectric constant
2. Halliday et al
• Believe fluctuating potential model is the most suitable for their PL
data
• PL more sensitive than XRD to impact of structural differences and
secondary phases
• More evidence for CZTS being a highly disordered system
• Possibly merge discussion of two effects (band gap and potential
fluctations) considered separately in the Gokmen study?

30. Fluctuating Potential Model

31. Original Figure For Radiative Recombination?
Local distortions
of band edges due
to fluctuations of
impurity potential
Wavy lines show:
band-tail (BT), band-
band (BB) and band-
impurity (BI) transitions

32. … The Verdict!

33. … The Verdict!

34. … The Verdict!
• Studies seemed to be well-
conducted
• Although I don’t know enough about
IQE to know how much to trust this
measurement!
• Interpretation of PL data seems to
be a very subjective process!
• Undecided if I agree with interpretations
in these papers
• Studies are well presented, arguments
clearly discussed and cite very useful
original works used in their analysis

35. … The Verdict!
• Studies seemed to be well-
conducted
• Although I don’t know enough about
IQE to know how much to trust this
measurement!
• Interpretation of PL data seems to
be a very subjective process!
• Undecided if I agree with interpretations
in these papers
• Studies are well presented, arguments
clearly discussed and cite very useful
original works used in their analysis

36. Resources
• Paper on PL of semiconductors:
http://ac.els-cdn.com/004060908190465X/1-s2.0-004060908190465X-
main.pdf?_tid=ec24517c-6c46-11e5-871b-
00000aacb35e&acdnat=1444148937_409d6ef7bc4d654e1841c5627b3b2da6
• Paper on band tailing:
http://journals.aps.org/rmp/abstract/10.1103/RevModPhys.64.755
• ‘Edge luminescence of direct gap semiconductors’ paper referred to by Mitzi
and Halliday in their interpretation of PL data:
http://iopscience.iop.org/article/10.1070/PU1981v024n03ABEH004770/meta
• Very good general background on band structure (perhaps not as much PL
spec as I would have liked!):
http://memo.cgu.edu.tw/sykuo/Mat-8.pdf
• Slides on electrical and optical characterization of semiconductors from NREL
(including various PL measurements):
http://www.nrel.gov/docs/gen/fy04/36831k.pdf